1. Field
The present application relates to a solid state imaging device.
2. Description of the Related Art
In a solid state imaging device of XY address type such as MOS type, generally, CDS (correlation double sampling) is performed to read a data level (including an original signal level and a noise level) and a noise level from pixels, and obtain a difference between them.
In such a solid state imaging device, if light with high luminance is incident when the noise level is read, the noise level output from pixels is a level such that a level based on the high luminance light is added to the original noise level. As a result, there is a small difference between the data level and the noise level, and a high luminance part as of the sun appears to be dark. Such a phenomenon is called a dark sun or the like.
To prevent such a phenomenon, in a solid state imaging device disclosed in Japanese Unexamined Patent Application Publication No. 2004-222273, a vertical signal line to which an output signal of pixels is supplied is provided with a clip transistor (which may also be referred to as a clamp transistor) which limits the potential of the vertical signal line based on a gate potential. To the gate of the clip transistor, different potentials are supplied when the noise level is read and when the data level is read. Accordingly, the noise level (reset signal) is limited to a predetermined potential. Therefore, a sufficient difference corresponding to the incident light can be obtained between the data level and the noise level, thereby preventing the aforementioned phenomenon.
Now, in the solid state imaging device, besides effective pixels which photoelectrically convert incident light to generate a signal corresponding to the incident light, optical black pixels generating a signal of a black standard level are provided. The optical black pixels are used for correcting the black level in a screen, or the like. The optical black pixels have, for example, basically the same structure as the effective pixels, but also have a structure in which a photoelectric conversion part, such as a photodiode, is shielded from light, or a structure in which the photoelectric conversion part is removed from the effective pixels. Optical black pixel areas are disposed at, for example, an upper end or lower end and a left end or right end of an effective pixel area. Regarding reading of the pixels, the optical black pixels and the effective pixels are handled in exactly the same manner.
To prevent the dark sun also in the solid state imaging device having the optical black pixels besides the effective pixels, similarly to the solid state imaging device of Japanese Unexamined Patent Application Publication No. 2004-222273, the clip transistor may be provided on the vertical signal line. At this time, in accordance with the common technical knowledge such that the optical black pixels and the effective pixels are handled in exactly the same manner with respect to reading of pixels, the clip transistor should be provided not only on the vertical signal line of a pixel column including the effective pixels but on the vertical signal line of a pixel column formed of the optical black pixels. Further, not only in a reading period of pixel rows including the effective pixels but also in a reading period of the pixel column formed of the optical black pixels, different potentials are supplied to the gate of the clip transistor between when the noise level is read and when the data level is read.
When the clip transistor is thus employed in the solid state imaging device having the optical black pixels in accordance with the common technical knowledge, no particular problem occurs unless a white defect (constantly outputting a level corresponding to a bright signal) occurs in the optical black pixels. However, as a result of studies conducted by the present inventor, if the white defect occurs in the optical black pixels when the clip transistor is employed as described above, this white defect becomes manifest. It was found that, as a consequence, the yield of solid state imaging devices decreases, and cost increase is inevitable. This point will be described later in detail with respect to a comparative example.
In addition, normally, the optical black pixels should always output a black level. However, it is empirically confirmed that the white defect occurs in optical black pixels with a constant probability.
The present application is made in view of such a situation, and it is a proposition of the application to provide a solid state imaging device capable of suppressing the white defect from becoming manifest when the white defect occurs in the optical black pixels, although it is also capable of preventing the dark sun, thereby leading to improvement in yield.